The present invention relates to plasma arc torches and, more particularly, to a method of forming an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys. Some electrodes include a relatively non-emissive separator, which is disposed about the emissive element and acts to prevent the arc from migrating from the emissive element to the copper holder.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element, and as the copper oxidizes, its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
The assignee of the present application has previously developed an electrode with significantly improved service life, as described in U.S. Pat. No. 5,023,425, the entire disclosure of which is hereby incorporated by reference. The ""425 patent discloses an electrode comprising a metallic tubular holder supporting an emissive element at a front end thereof, and having a relatively non-emissive separator or sleeve surrounding the emissive element and interposed between the emissive element and the metallic holder. In particular, the ""425 patent describes the fabrication of the metallic holder by axially drilling the separator and force fitting the emissive element therein. The resulting interference or frictional fit holds the emissive element in the separator, and the front face of the assembly is then finished to form a common front planar surface.
Processes have also been developed to increase the bond strength between the emissive element and the metallic holder. In particular, U.S. Pat. No. 5,200,594 describes a pressing process wherein the emissive element is coated with nickel and silver films, and then inserted into a metallic holder. The base of the electrode having the emissive element inserted therein is pressed from the periphery to the center by using pressing tools. The pressing process increases the bond between the films coating the emissive element and the metallic holder, which therefore improves the life span of the electrode.
The electrode and process of forming an electrode as described by the ""594 patent, however, increases the fabrication cost of the electrode due to the multiple film layers that must be applied in order to form a strong bond with the metallic holder. And electrodes according to the ""425 patent, although a great advance over prior electrodes, still have a life span that electrode manufacturers and users would like to see extended. Thus, there is a need to increase the life span and performance of an electrode without requiring extra or special coatings, films, or brazing materials to be applied between the emissive element, separator, and/or metallic holder.
The present invention was developed to improve upon conventional methods of making electrodes, and more particularly methods of making electrodes disclosed in the above-referenced ""594 patent. It has been discovered that the difficulties of the electrodes described above, namely increasing the life and performance of electrodes for plasma torches, can be overcome by providing an electrode by heating the electrode near the end of the manufacturing process to accelerate diffusion bonding between elements of the electrode. Advantageously, the xe2x80x9cpost-assemblyxe2x80x9d heating process forms stronger bonds between components of the electrode, which results in longer time and better performance of the electrode.
In particular, a method of fabricating an electrode according to the present invention includes forming an assembly by inserting an emissive element having a relatively low work function in a relatively non-emissive separator. The separator, which is formed of a metallic material having a work function greater than that of the emissive element, has inner and outer surfaces wherein the inner surface of the separator is in face-to-face contact with the emissive element. The assembly is positioned in a cavity defined by a metallic holder, the cavity being in surface-to-surface contact with the outer surface of the separator. After the assembly is in place, the metallic holder and assembly are heated to accelerate diffusion bonding between the emissive element and separator, and between the separator and the metallic holder.
The heating step comprises heating the metallic holder and the assembly to between about 1400xc2x0-1420xc2x0 F. for at least 5 hours and, more preferably, to about 1410xc2x0 F. for about 6.5 hours. In this manner, diffusion bonding, which also takes place at room or ambient temperatures but orders of magnitude slower, occurs relatively rapidly to increase the bonds between the emissive element and separator, and between the separator and the metallic holder. Because these elements are more secure, the Inventors have discovered that the life span of the electrode is greatly improved over conventional electrodes. In addition, brazing materials or coatings are not used according to the methods of the present invention, which thereby decreases the costs of manufacturing the electrode.
In a preferred embodiment, the heating process is followed by a crimping process, preferably after allowing the electrode to cool to ambient or room temperature. The crimping process includes using pressing tools to press the outer surface of the metallic holder radially inwardly towards the cavity defined therein in order to reduce the overall outer shape of the metallic holder. In one embodiment, the crimping process reduces the outer diameter or shape of the metallic holder by between about 0.050-0.100 inches, which is sufficient to add further strength and hardness to the electrode. The crimping process also substantially eliminates any voids present between the emissive element and the separator, and between the separator and the metallic holder that can lead to early failure of the electrode.
Thus, the present invention provides methods of making an electrode having stronger bonds between elements of the electrode, which improves the strength and operational life span of the electrode. Furthermore, the methods of making an electrode according to the present invention are directed to electrodes having no brazing materials, coatings, or other layers present between the emissive element, separator, or metallic holder. In this regard, the cost and complexity of fabricating the electrode is reduced.